The tree shrew visual system has been fertile ground for anatomical and behavioral investigations of the "first" (geniculostriate) and "second" (extrastriate) visual pathways from retina to cortex. It has a well-developed "second" visual pathway which mediates visual pattern discriminations following removal of the striate cortex. Our preliminary electrophysiological investigations have found three types of neurons in the superficial layers of the superior colliculus which may relay visual pattern information in the "second" pathway. We propose, therefore, to: (l) examine which of these receptive-field types may, in fact, serve as a "relay" in the extrastriate visual pathway; (2) assess the influence of the striate cortex on the receptive-field properties of these superior colliculus cells; (3) compare quantitatively the receptive-field properties of the superior colliculus relay cells with those of relay cells in the dorsal lateral geniculate nucleus; and (4) study the receptive-field properties of the "relay" cells in the pulvinar nucleus. We then propose to (5) study qualitatively and quantitatively the receptive-field properties of the neurons in the striate and extrastriate cortex, comparing the visual responses of the cells in the cortical targets of the "first" and "second" visual pathways. Finally, we will assess the effect of striate cortex removal on the neurons in the extrastriate cortex. These studies will provide important physiological data on the functioning of the extrastriate visual system which will lead to an understanding of its role in the mediation of normal pattern vision and particularly pattern vision following striate cortex removal.